Perforated hollow cathode discharge device



y 1966 R. J. ALLEN m, ET AL PERFORATED HOLLOW CATHODE DISCHARGE DEVICE Filed Feb. 2, 1962 2 Sheets-Sheet 1 8% Q q Q a INVENTORS. E. J. ALLEN 1E H L. L. 1/ PM WOF/Vfy July 19, 1966 R. J. ALLEN m, ETAL 3 2 2,013

PERFORATED HOLLOW CATHODE DISCHARGE DEVICE Filed Feb. 2, 1962 2 Sheets-Sheet 2 aw 4500 m V047W6E //v V0473 300 440 sac 600 7 1/04 7776f //v V0473 R J 4LLE/v I By H L.L. V/M/ lQmssszv United States Patent 3,262,013 PERFORATED HOLLOW OATHODE DISCHARGE DJEVTQE Richard ll. Allen Eli and Hugo L. L. van Paassen, Baltimore, Md, assignors to Martin-Marietta Corporation,

Baltimore, Md., a corporation of Maryiand Filed Feb. 2, 1962, Ser. No. 170,697 Claims. (Cl. 315-326) This invention relates generally to a perforated cathode and more particularly to a hollow cathode apparatus tor producing either narrow beam or diffused glow discharges in an ionizable gas at subatmospheric pressure.

Hollow cathode devices have been utilized in the past for producing glow discharges. Conventional hollow cathodes consist of a generally spherical hollow body provided with an aperture and formed from an electrically conductive material. When the hollow cathode body .is operated under certain conditions of voltage and pressure, a glow discharge is established that consists of a spherical glow centered within the cathode and a luminous column extending outwardly from this spherical glow through the cathode aperture. Once outside the cathode, the luminous column diverges (i.e., diffuses) rapidly.

This conventional discharge, which will be referred to in the following specification as being a diiiused glow discharge, produces voltage-current curves that are broader and flatter than those of a plane cathode discharge device. Measurements indicate that the glow region within the cathode cavity accounts rfor roughly 90% of the anode potential. Consequently, most of the potential drop occurs between the surface of the cavity and the glow region therein. Hollow cathode diff-used :glOW operation may be obtained over wide ranges of voltage and gas pressure. Various parameters affecting diffused glow discharge are the size of the cathode cavity, ionization potential of the gas, work function of the cavity wall, and diameter of the cathode aperture. Hollow cat odes having cavity diameters on the order of one centimeter operate in a pressure range of approximately 10 to 500 microns mercury, while one millimeter cavities operate at pressures from 1 to millimeters mercury. Voltages ranging 'from several hundred volts to several kilovol-ts are required to initiate the hollow cathode di ifused glow. At the higher voltages, however, sputtering of the cathode walls becomes excessive. This sputtering and the accompanying condensation of metal within the cathode changes the work function of the cavity walls and, consequently, the operating characteristics of the hollow cathode vary considerably with use. Attempts to focus the diffused glow discharge from conventional hollow cathode devices have proven unsuccessful.

One application of the known didused glow devices resides in the cleaning of substrates by heavy ion bombardment prior to condensation of thin layers of evapcrate-d metal thereon. It has been found that high energy electrons which are also present in gas discharges do not clean the substrate surfiaces but rather polymerize the residual oil vapor in the vacuum system, thereby producing a thin coating on the substrate.

The object of the present invention is to provide a hollow cathode device which is operable to produce either the conventional dill-used glow discharge described above or a focused, narrow beam discharge which hithertofore was unobtainable with hollow cathode discharge devices.

- The hollow cathode of the present invention is characterized by the provision of perforations in the cathode body adjacent the cathode aperture. In one embodiment of the invention described in the following specification, the cathode body is formed at least partially of conductive screen elements the mesh openings of which define the 3,262,913 Patented July 19, 1966 perforations necessary tor sustaining the narrow beam discharge.

Another object of the invention is to provide simple, inexpensive hollow cathode means tor generating an electron beam in an ionizable, subatmospheric gaseous medium without the use of heating filaments, focusing means, or external electrodes.

A further object of the invention is to provide a perforated hollow cathode that o-perates in the diffused glow m0de-at higher currents, over a greater pressure range, and with a greater range of aperture diameters, than conventional imperforate hollow cathodes.

Still another object of the invention is to provide a perforated hollow cathode discharge device that is readily s hifta ble between high and low impedance conditions when operated under appropriate conditions of pressure and voltage.

According to the present invention, a perforated hollow cathode is provided that produces, when operated within a given pressure range, either narrow-beam or diffused glow gaseous discharges. These two discharges differ from each other in appe-rance, electrical properties, nature of particles discharged, and eliect on t .e cathode surface. In the narrow beam mode of operation, the discharge has two distinct portions, one being a conventional spherical glow centered within the cathode cavity, and the other being a narrow focused beam of negatively charged particles originating just within the cathode aperture and extending outwardly therefrom. In the difiused glow mode of operation, the conventional discharge (consisting of a spherical glow centered within the cavity and a luminous column connected with the glow and extending through the aperture) is obtained. The spherical glow within the cathode cavity is much more intense during diffused glow operation than during narrow beam operation.

The perforated hollow cathode device of the present invention has utility as an electron beam generator, a display device, a source of sputtered metal, or an electron beam welding device. Other possible applications of the invention reside in electric space propulsion and microwave generation. Since the narrow beam comprises a column of charged particles, the perforated hollow cathode may also be used as an RF. transmitting or receiving antenna. Owing to its high discharge impedance and lessened cathode sputtering, the narrow beam mode of operation has particular merit in many gaseous discharge applications.

Other objects and advantages of the invention will become apparent from a study of the following specificaiton when considered in conjunction with the accompanying drawing in which:

FIG. 1 is a somewhat diagrammatic illustration of one embodiment of the invention;

FIG. 2 illustrates the manner in which the discharge current of the apparatus of FIG. 1 varies as a function of pressure (voltage being maintained constant) during narrow beam and diff-used glow modes of operation;

FIG. 3 illustrates the hysteresis curves obtained by varying the cathode voltage of a hollow cathode formed of copper screening; and

FIG. 4 discloses the manner in which the maximum discharge current varies as a function of voltage at various pressures for perforated hollow cathodes having apertures of difierent sizes. These curves are the locus of uppermost points of two families of curves as shown in FIG. 3.

FIG. 5 illustrates another embodiment of the invention wherein the cathode is provided with an aperture and two distinct sets of perforations.

Referring to the drawing, housing 11 (which in the illustrated embodiment constitutes a conventional vacuum bell jar) contains anode l2, cathode 13 and an ionizable gas (for example, helium) at subatmospheric pressure. Cathode 13 is of the hollow-cathode type having cylindrical Wall 14 and end walls 15 and 16. The diameter of end walls 15 and 16 equals the length of cylindrical wall 14 so that the cathode cavity is substantially spherical. Each of the cathode walls consists of wire screening formed from a conductive material, such as copper. Aperture 17 formed in cylindrical wall 14 has a diameter at least three to four times larger than the average screen mesh opening, and less than approximately one-third the cathode diameter. Anode 12 and cathode 13 are connected to the positive and negative terminals, respectively, of direct-current voltage source 18.

Operation Either narrow beam or diffused glow modes of discharge may be obtained by the apparatus of FIG. 1. Assuming that the cathode is formed from standard A by A inch copper wire screening and has a diameter of 4 cm., a length of cm., and a 0.75 cm. aperture, that the helium gas has an initial pressure on the order of 46 microns mercury, and that a potential of approximately 375 volts exists between the anode and cathode, a pinkish spherical glow is established in the cathode cavity and a bluish narrow beam 22 of negatively charged particles is emitted from aperture 17. Beam 22, which originates just within the aperture and is not an extension of the cavity glow, is tightly focused and has angle of divergency less than approximately three degrees.

Assuming now that the gas pressure is progressively increased, discharge current increases correspondingly as shown by curve a in FIG. 2. When the pressure reaches a transition pressure P, of approximately 85 microns, the discharge current increases instantaneously to a point on curve 1). Cathode 13 has now switched over to the diffused glow mode of operation, the increase in discharge current indicating a material reduction in discharge impedance. During this diffused glow mode of operation, a bluish spherical glow is centered within the cathode, and a luminous column extends from this spherical glow through the aperture. Once outside the cathode, the luminous column diffuses rapidly. This diffused glow is identical to that obtained with conventional imperforate hollow cathode discharge devices.

As gas pressure is progressively increased to a final pressure P discharge current increases correspondingly as shown by curve b. Cathode 13 continues to operate in the diffused glow mode. Assuming now that gas pressure is progressively decreased to a value lower than transition pressure P discharge current decreases proportionately along curves b and c; cathode 13 continues to operate in the diffused glow mode. When the extinction pressure Fe is reached, discharge current decreases instantaneously to zero. Upon increase in gas pressure above extinction pressure Pe, the narrow beam mode of operation is again initiated and discharge current increases linearly along curve a. When gas pressure exceeds transition pressure P cathode 13 again switches over to the diffused mode of operation as described above. Depending on system parameters, discharge current at transition may increase by a factor on the order of to 10 or more.

It is important to note that for gas pressure lying between extinction pressure and transition pressure, cathode 13 may be switched from one mode of operation to the other. Assuming that cathode 13 is operating in the diffused glow mode with a discharge current lying on curve "0, if the voltage applied across the anode and cathode should be momentarily interrupted, upon resumption of voltage, cathode 13 will operate in the narrow beam mode. On the other hand, cathode 13 may be switched from the narrow beam mode to the diffused glow mode by the application of high voltage radio frequency energy .to the discharge device. Thus at every combination of voltage and pressure at which the narrow beam mode of operation is obtained, the normal diffused mode may also be obtained. The converse is not true, however. For example, for gas pressures above transition pressure P only the diffused glow mode may be obtained.

For a given perforated hollow cathode, narrow beam operation is obtained at the lower end of the diffused glow pressure range. For a cathode having a diameter and length of two inches, diffused glow operation may be obtained well above 1,000 microns pressure and, with sufficient voltage, down to about 20 microns. The narrow beam mode of operation will occur for pressures between approximately 300 and 20 microns. If the gas pressure should be maintained constant (at a value permitting narrow beam operation) and the voltage should be increased progressively from zero, cathode 13 operates first in the narrow beam mode. As cathode voltage is progressively increased, discharge current increases correspondingly until a transition voltage is reached that results in an instantaneous increase in current by a factor of from approximately 10 to 10 or more. This discontinuity in current marks the transition from the narrow beam mode to the diffused glow mode. If the voltage is now decreased, the diffused glow mode is continued until extinction voltage is reached. Should the voltage be interrupted during diffused glow operation, narrow beam discharge is obtained upon voltage resumption as described above.

If the potential between anode and cathode should be increased (for example, to 500 volts), the transition and extinction pressures are lowered (i.e., displaced to the left in FIG. 2) and the pressure range between them is decreased. Increase in voltage results in an increase in beam focusing and maximum obtainable beam current.

In addition to the differences in appearance and impedance characteristics, the two types of discharges also have different effects on the cathode surface. The conventional diffused glow discharge causes considerable sputtering and rapid cleaning of the cathode surface. On the other hand, the narrow beam discharge does not clean the cathode surface to any appreciable extent, and under certain conditions of voltage and pressure, actually effects coating of the cathode surface. Consequently, cleaning of a cathode operating in the narrow beam mode may be accomplished by momentarily switching cathode operation to the diffused glow mode.

Theory Successful operation of the hollow cathode in the narrow beam mode is dependent upon the provision of perforations in the cathode body adjacent aperture 17. It is believed that the theory behind the narrow beam mode of operation may be explained as follows:

Magnetic beam deflection indicates that the particles of beam 22 are negatively charged and have an energy on the order of to of the potential between anode and cathode. In view of their masses, it would appear that the particles are electrons. It is estimated that all particles have the same energy within 10% (i.e., that the particles have an energy spread less than about 10%).

Assuming that the potential between anode and cathode causes ionization of the gas, electrons are attracted to anode 12 and ions are attracted to cathode 13. Some of the ions attracted to the cathode impinge against the cathode wall while others enter the cathode cavity via the perforations and the aperture. Within the cavity the ions are neutralized by electrons emitted from the cathode and produce a spherical glow (resulting from the visible light produced as the electrons pass to the lower energy level). Under certain conditions of voltage and pressure, excess free electrons are produced in the cavity by cathode emission, by collisions of ions with neutral gas particles, and by secondary emission as ions bombard the cavity wall. Owing to electric field perturbation adjacent aperture 17, the free electrons flow toward the aperture and (as a consequence of cathode potential and the mutual repulsion of the electrons) are emitted therethrough in the form of a high energy narrow beam. A positive glow, which enters the cathode cavity through the aperture during diffused glow operation, appears adjacent the anode in the form of a well defined column indicated diagrammatically by reference numeral 23 in FIG. 1. It is a believed that during narrow beam operation, most of the gas (with the exception of the beam and the positive column) acts as a dielectric. In the case of the conventional diffused glow mode of operation, it is believed that a substantially larger volume of the gas in the vacuum chamber acts as a conducting plasma.

The narrow beam discharge is sustained by the flow of ions into the cavity via the perforations (and to some extent, via the outer peripheral portion of the aperture). Owing to the intensity of the beam at the aperture, the number of ions entering the cavity through the aperture is rather small.

The shape and cross-section of the beam may be varied by appropriate adjustment of voltage and pressure. Under suitable high voltage conditions, the beam may become so intense that a protective plate 24 must be provided adjacent the upper portion of bell jar 11 to protect the same against damage by the beam.

When the perforated hollow cathode is switched over to the diffused glow mode of operation, the generation of narrow beam 22 ceases and the intensity of the spherical glow within the cathode cavity increases. The discharge current during diffused glow operation is greater than that obtained with conventional hollow cathodes since the perforations afford additional passages for the entry of ions into the cavity.

Parameters Many parameters affect the operation of the perforated hollow cathode discharge device.

One of the main variables affecting discharge current and shape of the beam is cathode potential. This variable may be accurately controlled by power supply adjustment. Adjacent the transition point between narrow beam and diffused glow modes of operation the voltage current curve is very steep; a small increase in voltage produces a large increase in current. Consequently, at high power levels it is difficult to avoid overshooting and losing of the beam mode. Although the voltage source 18 has been described as being a direct-current source, it is obvious that suitable alternating current sources may be used as well. In this case, only the negative half cycles of the voltage cause emission of electrons from cathode 13.

As described above, gas pressure is another important factor affecting narrow beam discharge. This parameter is more difficult to control. At the higher power levels, a small increase in pressure generally causes the narrow beam mode to switch to the diffused glow mode. It is at the higher power levels that both the cathode and the target of the beam are heated and outgassed. Cathodic sputtering with the resultant release of trapped gases also adds gas to the system. Under these conditions it is difficult to maintain constant gas pressure. Furthermore, the maximum voltage which can be used and still maintail narrow beam operation, drops sharply with increasing gas pressure.

Changes in gas composition (which result in displacement of the voltage-pressure-current curves) are difficult to avoid in hollow cathode discharge devices. In the case of a gas mixture, certain components are removed during use by entrapment on the cathode walls. Pure gases become contaminated by outgassing and sputtering, thus varying ionization potential. Since the perforated hollow cathode exhibits relatively low sputtering during narrow beam operation, it is apparent that the present invention offers the advantage of minimm contamination of the gas. Both helium and nitrogen have been used successfully, helium being preferable since it produces a higher discharge current.

A decrease in the size of the cathode displaces the voltage-current-pressure curves in the direction of higher pressure and extends the pressure range over which the narrow beam mode will operate. The shape of the oathode is important. As has been noted above, the cylindrical screen cathode does not define a perfectly spherical cavity. At certain voltage and pressure combinations, the beam assumes an elliptical cross-section the major axis of which is either parallel or normal to the cylinder axis. This shows that beam formation is dependent on the shape of the cathode. Distortion of the beam might be limited by the use of an absolutely spherical cavity. Beam cross-section is also a function of cathode voltage.

Aperture diameter, cavity diameter, screen gauge, and pressure similarly affect narrow beam operation. The aperture diameter should be at least 3 to 4 times greater than the mesh openings to avoid leakage of the discharge through the cathode walls.

The material and surface condition of the cathode are additional important parameters. In actual practice, beam current was found to vary as a function of a combination of time drift and hysteresis factors. The time drift component may be eliminated by operating the perforated hollow cathode for a short period of time in the diffused glow mode at high current levels, i.e., 0:5 to 1.0 amperes. This operation causes surface contaminants to be cleaned away by cathode sputtering. Hysteresis effects were found to be present when voltage is varied under constant pressure conditions (see FIG. 3), the size of the hysteresis loops being dependent on the extent of the voltage swing. Cathodes formed from copper screening were found to exhibit larger hysteresis loops than those formed from aluminum screening. Since the hysteresis effect is dependent, at least in part, on the release of trapped gas during sputtering, the use of metals having low sputtering characteristicssuch as tungsten, for example-for the cathode screen material is desirable.

The application of a magnetic field not only deflects the beam but also affects the pressure-voltage operating range of the beam mode, generally reducing the range. If the magnetic field is increased so that the beam is bent into a full circle returning to the cathode, the narrow beam discharge is extinguished. The beam has also been deflected by electric fields. Normally the beam is operated so that it is directed upward toward the dome of the bell jar of the vacuum system. The base plate of the system is at ground potential, and consequently the beam is directed against the expected electric field. If a wire grid at ground potential is placed a few inches above the cathode in the path of the beam, the faint glow in the upper part of the vacuum chamber is somewhat reduced. A phosphor screen placed inside the dome of the bell jar will also show a sharp image of the screen; only at the upper part of the voltage range is this image distorted. At these high voltages the image of the screen is enlarged by a factor of two or three. It has been found that the same imaging effect occurs if the screen is left ungrounded and floating. Negative potentials up to of the cathode potential have little apparent effect on the beam for normal angle of incidence. The beam itself has more than sufiicient energy to produce secondary electron emission from any surface which has been placed in its path and, in some cases, actually evaporates the surface. Glass surfaces under high power conditions have been noted to incandesce and shatter.

The region of conducting plasma is substantially different in the beam modes from the hollow cathode mode. The cathode surface is for all intents and purposes, an equipotential surface with an anomaly at the aperture. This surface will permit the flow of gas and ions and to some extent, electrons, through the surface. The entire surface of the cathode is surrounded by a cathode dark space characteristic of a glow discharge. The cathode glow is observed on both the interior and exterior surfaces of the cathode. It is suggested that the positive ions and photons which bombard the surface of the cathode produce electrons on both the interior and exterior surfaces of the cathode. The result is that a substantial fraction of the electrons thus produced find themselves on the interior of the cathode and are repelled by the screen walls. In a very short time after the initiation of the discharge a large surplus of electrons is produced Within the cathode and will tend to leak out through the most favorable hole in the cathode potential surface. An aperture larger than the screen mesh provides such a hole. Once the electrons leak through the aperture they are rapidly accelerated as they pass through the cathode dark space.

Dark mode of narrow beam operation As mentioned above, for cathodes of the same size, the current and pressure values for perforated hollow cathodes operating in the diffused glow mode are higher than for conventional imperforate hollow cathodes. The low-current narrow beam mode was found to occur at the lower range of pressure and voltage. lit has been found that by further decreasing pressure, the beam voltage and currents may be increased to higher values without reverting to the diffused glow mode. A dark beam mode of operation has also been noted, which will be described with reference to numerical data obtained on one specific cathode geometry. At pressures on the order of 10 to microns, the narrow luminous beam currents reach a sharp maximum. Below this pressure, a discontinuity occurs in the pressure-voltage-current domain. At voltages starting at about 1800 volts and pressures about 10 microns, a current of several milliamperes is produced. This current in itself is distinctive since the impedance is 5 to 10 times greater than impedance values at pressures a few microns higher. Furthermore, there exists a marked decrease in the visible intensity of the discharge. By means of a phosphor screen, it was determined that a broad invisible electron beam is directed from the aperture toward the top of the vacuum chamber. The theory behind this dark mode of operation is not known. It should be mentioned that the dark mode shows the same characteristic are the beam mode in that it will degenerate discontinuously if the voltage is increased too far. The discontinuity is much more violent and is accompanied by a brilliant flash of light. It is suspected that the dark mode degenerates into an are at the higher voltages.

Experimentation reveals that for perforated hollow cathodes having apertures of different sizes, maximum current values occur at the same voltage and pressure (FIG. 4). One cathode produced this maximum current while operating with a brilliant intense beam. The other cathode, with the larger aperture, produced the maximum current while operating in the dark mode. The combination of voltage-pressure and cavity diameter, independent of aperture and operational mode, produces maximum current values at the same point. This result is strongly indicative of a resonance effect within the cavity. It was found that for a 6.5 cm. x 6.5 cm. aluminum mesh cathode operating in a helium atmosphere, maximum current occurred at approximately 1850 volts for cathodes having aperture diameters of 0.65 cm. and 1.30 cm. respectively.

Possible applications One possible application of the narrow beam discharge device is as a display device. It is possible to perform various demonstrations of electric and/or magnetic field deflection with a visible electron beam. The beam cross section can be easily demonstrated by its luminous track or with the aid of a phosphor screen placed in the vacuum chamber. This cross-sectional shape can be varied by suitable variation in cathode voltage.

The perforated hollow cathode may also be used as a source of sputtered metal. The cathode material is fairly rapidly sputtered at the higher power levels. Glass plates placed in the vicinity of the cathode rapidly become coated with the cathode material.

Since the beam is a charged column, another application of the invention is use as an antenna. It is not known if the beam can be modulated at high frequencies, but the beam will work well when high voltage at 60 cycles per second is applied to the cathode. The beam has also been modulated at frequencies up to and above 5 mc. with the modulating signal applied to a conducting mesh in the beam, exterior to the cathode. The maximum modulation frequency is determined by the transit time of electrons along the beam, said transit time being a function of cathode potential.

The narrow beam apparatus may also be used as an electron beam welder. With a cathode voltage of 3600 volts, the narrow beam is energetic enough to cause glass to incandesce and shatter. Most electron beam welding devices utilize 20,000-volt accelerators. With such potentials, the beam produced by the perforated hollow cathode device would be quite intense and powerful. At 3000 volts and 15 milliamperes, the whole system consumes only watts.

When the beam is operated at relatively low power levels and is directed against a glass plate, a deposit is formed which appears to be metallic. As yet the composition of this deposit is not known. However, electrons have been observed to interact with other materials in a vacuum system to form deposits. Even though most of the particles in the beam are electrons, it is possible that heavier ions are also present in smaller numbers. If the heavier ions are present in sufiicient quantities, this beam discharge would be of possible interest in the field of electric space propulsion.

In view of the apparent resonance effects and the electric field distribution within the cathode cavity, the perforated hollow cathode operating in the narrow beam mode has possible utility as a microwave generator. Beam distortion indicates that at certain voltage pressure combinations, the oscillations are polarized but not neces sarily coherent.

Referring now to the embodiment of FIG. 5, the cylindrical hollow cathode 31 is formed from conductive screening and includes an opening 32 in which is mounted ring 33 by radial supports 34. Both the ring and the supports are formed from an electrically conductive material. In this embodiment, the opening 35 defined by the ring constitutes the aperture, the openings 36 defined between the radial supports constitute a first set of perforations immediately adjacent the aperture, and the screen mesh openings constitute a second set of perforations remote from said aperture relative to the first set.

Although we have illustrated and described the best embodiment of the invention now known to us, it will be apparent to those skilled in the art that modifications may be made in the apparatus described without deviating from the invention set forth in the following claims. For example, instead of forming the entire cathode from conductive screening, only the wall portion containing the aperture need be perforated. Perforations of various sizes and shapes may be provided in hollow cathodes formed from sheets of conductive material in accordance with the present invention. The perforations surrounding the aperture are most important for the maintenance of the beam mode of discharge. It is not essential that the perforations immediately surrounding the aperture be smaller than the aperture, although elsewhere they must be smaller. If several holes of equal size are used, one or more beams may be produced simultaneously. That hole which serves as the effective aperture is dependent on the spacing of the holes and the placement of the anode. If the holes are symmetrically placed relative to each other and with respect to the cathode surface, generally the central hole, if any, will serve as the aperture.

What is claimed is:

1. A gas discharge device comprising a housing; I

an ionizable gas in said housing, said gas being at a subatmospheric pressure;

an anode and a cathode positioned in spaced relationship in said housing, said cathode containing a cavity and having a portion provided With at least one aperture and with a plurality of perforations adjacent to said aperture,

said aperture and said perforations opening into said cavity; and means for establishing a potential difference between said cathode and said anode to cause emission of a narrow beam of negatively charged particles outwardly from said aperture.

2. Apparatus in accordance with claim 1 wherein the diameter of said aperture is at least three times as large as the average diameter of said perforations.

3. Apparatus in accordance with claim 1 wherein at least a portion of said cathode comprises a screen of electrically conducting material that defines said cavity at least in part, and wherein the mesh openings of said screen comprise said perforations.

4. Apparatus in accordance with claim 3 wherein the diameter of said aperture is at least three times as large as the average diameter of said perforations.

5. Apparatus in accordance with claim 1 wherein said cavity is substantially spherical in shape.

6. Apparatus in accordance with claim 5 wherein at least a portion of said cathode comprises a screen of electrically conducting material that defines said cavity at least in part, and wherein the mesh openings of said screen comprise said perforations.

7. Apparatus in accordance with claim 5 wherein the diameter of said aperture is at least three times as large as the average diameter of said perforations.

8. Apparatus in accordance with claim 7 wherein at least a portion of said cathode comprises a screen of electrically conducting material that defines said cavity at least in part, and wherein the mesh openings of said screen comprise said perforations.

9. Apparatus in accordance with claim 5 wherein the diameter of said cavity is at least three times as large as the diameter of said aperture.

10. Apparatus in accordance with claim 5 wherein at least a portion of said cathode comprises a screen of electrically conducting material that defines said cavity at least in part, and wherein the mesh openings of said screen comprise said perforations;

the diameter of said aperture is at least three times as large as the average diameter of said perforations; and

the diameter of said cavity is at least three times as large as the diameter of said aperture.

11. Apparatus in accordance with 'claim 5 wherein said cathode comprises a right circular cylinder of electrically conductive screening, with the diameter of said cylinder being substantially equal to its length.

12. Apparatus in accordance with claim 1 wherein said perforations comprise first and second sets of perforations,

said first set of perforations being located immediately adjacent to said aperture and 'being of a size comparable to the size of said aperture, and

said second set of perforations being located more remotely from said aperture and being of substantially smaller size than said aperture.

13. Apparatus in accordance with claim 1 wherein at least a portion of said cathode comprises a screen of electrically conducting material that defines said cavity at least in part;

said screen defines an opening;

a conductive ring is arranged within and spaced from the edges of said opening; and

conductive means connects said ring with said screen.

14. Apparatus in accordance with claim 1 wherein the subatmospheric pressure of said ionizable gas is in the range of microns of mercury.

15. Apparatus in accordance with claim 1 wherein the subatmospheric pressure of said ionizable gas is within the range of 1 to 2000 microns of mercury.

References Cited by the Examiner UNITED STATES PATENTS 2,419,236 4/1947 Stutsman 313-346 X 2,429,118 10/ 1947 Besser 313-211 2,504,224 4/1950 Patriarche 313-348 X 2,530,990 11/ 1950 Peters 313- 348 X 2,810,090 10/ 1957 MacNair 313-69 JAMES W. LAWRENCE, Primary Examiner. ARTHUR GAUSS, GEORGE N. WESTBY, Examiners.

L. D. BULLION, R. SEGAL, P. C. DEMO, Assistant Examiners. 

1. A GAS DISCHARGE DEVICE COMPRISING A HOUSING; AN IONIZABLE GAS IN SAID HOUSING, SAID GAS BEING AT A SUBATMOSHPERIC PRESSURE; AN ANODE AND A CATHODE POSITIONED IN SPACED RELATIONSHIP IN SAID HOUSING, SAID CATHODE CONTAINING A CAVITY AND HAVING A PORTION PROVIDED WITH AT LEAST ONE APERTURE AND WITH A PLURALITY OF PERFORATIONS ADJACENT TO SAID APERTURE, SAID APERTURE AND SAID PERFORATIONS OPENING INTO SAID CAVITY; AND MEANS FOR ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN SAID CATHODE AND SAID ANODE TO CAUSE EMISSION OF A NARROW BEAM OF NEGATIVELY CHARGED PARTICLES OUTWARDLY FROM SAID APERTURE. 